ath9k: fix an RCU issue in calling ieee80211_get_tx_rates

ath_txq_schedule is called outside of the drv_tx call, so it needs RCU
protection.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -1570,6 +1570,8 @@ void ath_txq_schedule(struct ath_softc *sc, struct ath_txq *txq)
 	    txq->axq_ampdu_depth >= ATH_AGGR_MIN_QDEPTH)
 		return;
 
+	rcu_read_lock();
+
 	ac = list_first_entry(&txq->axq_acq, struct ath_atx_ac, list);
 	last_ac = list_entry(txq->axq_acq.prev, struct ath_atx_ac, list);
 
@@ -1608,8 +1610,10 @@ void ath_txq_schedule(struct ath_softc *sc, struct ath_txq *txq)
 
 		if (ac == last_ac ||
 		    txq->axq_ampdu_depth >= ATH_AGGR_MIN_QDEPTH)
-			return;
+			break;
 	}
+
+	rcu_read_unlock();
 }
 
 /***********/